A system which performs defrosting by introducing hot gas into an evaporator at the defrost time is previously known as shown in the specification and drawings of U.S. Pat. No. 4,353,221. To explain this conventional system in FIG. 12, a three-way valve TV is provided on the high pressure gas line B of a compressor A, one outlet of said three-way valve being connected to a condenser C and the other outlet to a hot gas by-pass passage H bypassing said condenser C, receiver R and expansion valve EV, said hot gas by-pass H being connected to the inlet side of said evaporator E, said hot gas by-pass passage H being provided with a pressure regulating valve V.sub.1 which throttles its opening by sensing the pressure rise at the outlet side of said evaporator E, a pressure regulating valve V.sub.2 which opens by sensing the increase in high side pressure being provided between said hot gas bypass passage H and said condenser C. In the defrosting operation, said three-way valve TV is switched on to the hot gas bypass passage H to use hot gas in said evaporator E for defrosting and said two pressure regulating valves V.sub.1, V.sub.2 control their respective openings so that neither suction pressure nor discharge pressure does not rise abnormally.
With this conventional system, however, in case of overloaded defrosting operation, though ot gas quantity passed through the hot gas bypass passage H to the evaporator is controlled by said pressure regulating valves V.sub.1, V.sub.2, the surplus hot gas is bypassed, through said pressure regulating valve V.sub.2, into the condenser C and the receiver R and in liquid form, flows into said evaporator E together with said hot gas. In other words, with this system, the refrigerant quantity charged into the system circulates at the defrosting operation and the defrosting heat amount of hot gas is reduced by the amount corresponding to the refrigerant quantity bypassed to the condenser C. In spite of no decrease in the compressor A input, defrosting heat amount is decreased, which results in that much costly and inefficient defrosting operation.
Conventionally, a refrigeration system which has a hot gas bypass passage to supply hot gas discharged from the compressor to an evaporator, bypassing a condenser and controls its capacity for holding the hold temperature in the chilled range by adjusting hot gas quantity bypassed to said evaporator has been disclosed, for example, as shown in the specification and drawings of U.S. Pat. No. 3,692,100.
To explain the outline of this conventional system in accordance with schematic drawing, FIG. 13, a hot gas bypass passage is connected to the high pressure gas line which connects the discharge side of a compressor A with the inlet side of condensers C.sub.1,C.sub.2 so as to bypass said condensers C.sub.1,C.sub.2, a receiver R and expansion valve EV, said hot gas bypass line H being connected to the inlet side of the evaporator, said hot gas bypass line H being provided, near at its connection to said high pressure gas line B, with a hot gas valve HV which controls hot gas bypass quantity to said evaporator E, the capacity of said evaporator E being controlled by adjustment of said hot gas valve HV so as to control the supply air temperature consequently, the hold temperature in the chilled range.
By the way, with the conventional system when said evaporator E is fronsted, the defrosting operation performed by circulating hot gas through said evaporator E may be adopted and performed. Generally in case of cold storage operation for controlling the hold temperature in the chilled range, the low side pressure of refrigerant becomes high and the refrigerant circulation quantity becomes that much larger and on the other hand, in case of refrigeration operation for controlling the hold temperature in the refrigeration range, the low side refrigerant pressure becomes lower and the refrigerant circulation quantity becomes small. For this reason, in case of defrosting operation by hot gas, the refrigerant circulation quantity around the defrosting circuit varies with the operating condition immediately before entering defrosting operation, which results in the following problems.
That is, when the defrosting operation is entered from a cold storage operation condition wherein the low side pressure of refrigerant is high and refrigerant circulation quantity is large, it is possible to complete defrosting in a short time because of large refrigerant circulation around the defrosting circuit, but on the other hand, because of high air temperature around said evaporator E, refrigerant pressure becomes abnormally high when returning to the cold storage operation and overloads the compressor motor, which results in bringing the system beyond the operation range and the failure to run the system by the operation of the high pressure switch and the over-current relay. Further, when the defrosting operation is entered from a refrigeration operation condition wherein the low side pressure is low and refrigerant circulation quantity is small, it takes long to complete defrosting because of small refrigerant circulation around the defrosting circuit.
As stated above, when conducting defrosting by passing hot gas through the evaporator E, the hot gas circulation quantity through said evaporator E varies with the operating condition immediately before said defrosting, which makes an appropriate defrosting operation impossible.